1. Field of the Invention
The present invention relates generally to radiotelephones and radio telephone systems and, in particular, but not limited to, to time division multiple access (TDMA) cellular radio telephones, mobile stations radio communication systems and networks; and, furthermore, to GERAN based multi-carrier (MC) packet data systems that employ retransmission techniques, e.g., incremental redundancy (IR) and Universal mobile telecommunications Terrestrial Radio Access Network (UTRAN) systems; where GERAN is an composite acronym for Global Systems for Mobile communications (GSM)/Enhanced data rates for GSM Evolution (EDGE) Radio Access Network (GERAN).
2. Prior Art
It is known in the art to code information data bits with redundant code bits in order to improve the probability of a receiver receiving the information data. It will be appreciated that the higher the code rate, the ratio of redundant code bits to information bits, the lower the information data throughput and spectral efficiency.
Methods to improve the probability of reception include the use of Forward Error Correction (FEC) transmission techniques. FEC coding redundantly codes each bit to allow a receiving decoder to detect and correct transmission errors. FEC coding has several advantages over other types of error correction, such as automatic request for transmission (ARQ). For one, FEC methods do not require that the data be retransmitted before the data is received without error since FEC methods rely upon the higher probability that the information data can be successfully retrieved by the receiver's FEC decoders. This can greatly improve throughput of the transmission link. Also, since the receiver does not have to request retransmission it is possible to use one way or simplex data links and receive-only terminals or devices.
Yet, FEC decoders have a limit as to the number of errors within a transmission for which they can correct. Thus, a burst transmission traveling over a channel may experience degradation across a substantial portion of the burst length, degradation for which a FEC decoder may not be able to correct for.
In addition, it will be readily appreciated that the spectral efficiency of a communication system may be decreased with FEC coding since each information bit is redundantly coded to make the information bit more impervious to transmission errors.
As noted above, another method to ensure reception is for the receiver to automatically request retransmission (ARQ) if the data is not received or has errors. However, methods using ARQ requires a more complex receiver, e.g., a transceiver, and duplex data links. It will be appreciated that it is known to combine FEC methods with ARQ methods to produce hybrid ARQ-FEC systems. In general there are two types of hybrid ARQ systems. One type discards the erroneously received data and requests retransmission until the data is correctly received or until a pre-determined number of retransmissions has occurred. Another type stores the erroneously received data, and, using optimization techniques combines it with the next retransmitted and re-received data until the data is error free. In general, subsequent re-transmissions of this second type of hybrid ARQ method exploit incremental redundancy techniques.
Incremental redundancy (IR) techniques are based on a receiver initiated request to the transmitter to retransmit data. The transmitter then reduces the redundancy code rate, encodes the data to be retransmitted at the lower code rate, and retransmits; or transmits a different set of parity bits at the same code rate. This procedure is continued until the data is correctly received, or until a predetermined criteria has been met, e.g., a time limit has been reached.
One such form of redundant codes used to encode the information data is called puncturing codes. Puncturing codes are well known in the art and need not be discussed here. Referring to FIG. 1, there is shown a graph illustrating an example downlink performance with IR puncturing codes P1-P3. As seen, the performance of puncturing code P1 (first transmission) is poor. After the second and third retransmission at puncturing code rates P2 and P3, respectively, (decoding with P1+P2 and P1+P2+P3) the cumulative bit error rate of an individual radio link control (RLC) block may be improved, but at the expense of system throughput and delay.
Therefore, it is desirable to provide a method and system for a communication system that overcomes the disadvantages noted above.